Carbon fibre production

ABSTRACT

A process for the preparation of polyacrylonitrile precursor fibres and their subsequent conversion to carbon fibres, in which at least the fibre spinning process and oxygen permeation process are carried out under conditions in which particles, and so far as the spinning solution is concerned air bubbles, are excluded from the liquids employed in the spinning process and from the gases in which such process and the oxygen permeation process take place is described. Carbon fibres produced by the above process have an ultimate tensile strength which increases as the final heat treatment temperature is increased, over the whole range of final heat treatment temperatures used up to 3000*C.

United States Patent [191 Moreton et al.

[ CARBON FIBRE PRODUCTION [75] Inventors: Roger Moreton, ChurchCrookham; William Watt, Farnborough, both of England National ResearchDevelopment Corporation, London, England 221 Filed: Apr. 4, 1974 21Appl. No.: 458,035

[73] Assignee:

[30] Foreign Application Priority Data Apr. 6, 1973 United Kingdom16546/73 AVERAGE TENSILE STRENGTH X|O |bs./sq.in 3

[451 Sept. 9, 1975 Primary Examiner-Jay H. Woo Attorney, Agent, orFirm-Cushman, Darby & Cushman [5 7] ABSTRACT A process for thepreparation of polyacrylonitrile precursor fibres and their subsequentconversion to carbon fibres, in which at least the fibre spinningprocess and oxygen permeation process are carried out under conditionsin which particles, and so far as the spinning solution is concerned airbubbles, are excluded from the liquids employed in the spinning processand from the gases in which such process and the oxygen permeationprocess take place is described. Carbon fibres produced by the aboveprocess have an ultimate tensile strength which increases as the finalheat treatment temperature is increased, over the whole range of finalheat treatment temperatures used up to 3000C.

12 Claims, 2 Drawing Figures FIG.I.

HEAT TREATMENT TEMPERATURE C CARBON FIBRE PRODUCTION The presentinvention is concerned with the production of carbon fibres.

Processes for the production of carbon fibres are known, for example, UKSpecification No 1,110,791 discloses the conversion of polyacrylonitrileto carbon fibre by heating at a temperature in the range 200250C in anoxidising atmosphere for a time sufficient to permit complete permeationof oxygen followed by carbonisation at a temperature'of at least 1,000Cwherein the fib e is subjected to tension at least at some stage in itsconversion to carbon fibre. The process disclosed in this specificationalso contemplates a further heat treatment at a temperature of up to3,000C. The process of oxygen permeation is frequently termed oxidation;carbonisation and further heat treatment may be separate or the fibremay be passed as a continuous tow from one furnace to another at theappropriate temperatures.

Carbon fibres produced by this process and by modifications of thisprocess as hereinbefore described are disclosed in,for example UKSpecifications 1,168,619, 1,165,252, 1,166,251, show a Youngs Moduluswhich increases as the final heat treatment temperature is in creasedbut the ultimate tensile strength shows a maximum, generally in theregion of 1,500C. It is therefore impossible to obtain a carbon fibre bythese processes having simultaneously maximum values of both YoungsModulus and ultimate tensile strength.

In accordance with the present invention a process for the preparationof polyacrylonitrile precursor fibres and their subsequent conversion tocarbon fibres which includes the steps of spinning the polyacrylonitrileprecursor fibres from solution, heating the precursor fibres at atemperature in the range 200300C in an oxidising atmosphere for a timesufficient to permit complete permeation of oxygen while the naturalshrinkage of the polyacrylonitrile precursor fibre is at leastrestrained, followed by carbonisation and further heat treatment attemperature of up to 3,000C includes the improvement wherein thespinning process and oxygen permeation process are carried out underconditions in which particles, and also in the case of the spinningsolution, gas bubbles, are excluded from the liquids employed in thespinning process and from the gases in which such process and the oxygenpermeation process take place, whereby carbon fibres are produced havingan ultimate tensile strength which increases as the final heat treatmenttemperature is increased over the whole range of final heat treatmenttemperatures.

Advantageously the carbonisation and also the further heat treatment arealso out under conditions in which particles are excluded.

Generally the particles are excluded by filtering the said liquids andgases'through filters capable of removing any particles having a sizegreater than 3 microns.

Preferably the filtering applied to the liquids is such that the size ofparticles excluded is the smallest commensurate with ease of filtration.The less viscous the liquid being filtered the smaller the particleswhich can be easily removed. For example with spinning solutions whichare relatively viscous it had been found convenient to use a filtercapable of excluding any particles having a size greater than 1.5microns, whereas less viscous coagulation bath liquids and wash liquidscan be conveniently filtered through a filter, capable of exeludingparticles having a size greater than 0.25 microns.

The air or gas supplied to air or gas spaces around the apparatus isadvantageously passed through laminar air flow filters capable ofmeeting Class clean room conditions as set forth in U.S. FederalStandard 209A, that is not more than 100 particles per cubic foot of asize greater than 0.5 microns and none greater than 5 microns, andpreferably such gas or air does not contain more than 10 particles percubic foot of a size greater than 0.5 microns.

Preferably where a stage of the process, eg the oxygen permeation step,carbonisation or further heat treatment is carried out in a stream ofgas, that gas is passed through a 0.05 micron filter before contact withthe fibre under heat treatment.

In the present specification carbonisation means heating in vacuo, or inan inert of reducing atmosphere with respect to carbon, at a temperatureat which volatile materials are driven off from the polyacrylonitrilefibres leaving a carbon residue which may contain a minon proportion ofother elements, eg up to 5% by weight of nitrogen. The higher thecarbonisation temperature, the lower the nitrogen content of the finallyproduced carbon fibres is. For example, at 1000C about 5% by weight ofnitrogen remains while at 1500C substantially all the nitrogen is drivenoff. Carbonisation takes place at temperatures broadly within the range800 to 1200C although temperatures of up to 1500C may be included.

Further heat treatment may be an extension of the carbonisation processin which the temperature is raised to the described final temperature orit may be a separate step or steps.

The term polyacrylonitrile as used in the present specification includeswithin its scope copolymers or terpolymers or acrylonitrile with notmore than 15% and preferably less than 10% by weight of other monomers,for example, methyl methacrylate, methyl acrylate or vinyl acetate,either alone or to which have been added polymers compatible with them.

It has been found that carbon fibres having a length of less than 5 cm,produced in accordance with the present invention do not show asignificant change in tensile strength as the gauge length of the testspecimen is reduced.

The spinning of polyacrylonitrile precursor fibres and their conversionto carbon fibres in accordance with the present invention will now bedescribed by way of example only, together with the spinning andconversion of polyacrylonitrile fibres as a control.

The spinning apparatus was a laboratory spinning apparatus. Theapparatus included a reservoir for the spinning solution pressurised byargon and a stainless steel spinneret. After extrusion the fibre passedsequentially through a coagulation bath, 1.20 metres in length, a waterwash bath, a steam stretch tube, 0.60 metres long, a filrther water washbath, a traversing device, and was finally taken up on a fused silicacollecting frame. All the baths were contained in polyethylene coatedstainless steel tanks.

A bank of laminar air flow filters directed a flow of clear air towardsthe apparatus. The flow was directed in a direction parallel to thelongitudinal axis of the spinning apparatus with the mechanism of thespinning apparatus and the operators downstream of the apparatus so thatany contamination generated by them was carried away from the apparatus.

In the vicinity of the steam stretch tube the laminar air flow wasdirected downwards and an extractor was provided below the surface onwhich the steam stretch tube was supported It is contemplated thatstretching in hot glycerol at temperatures above 100C, e.g. 150C, couldbe substituted for steam stretching.

The laminar air flow filters were nominally capable of providing a cleanzone covering the apparatus up to Class 100 conditions as set forth inUS Federal Standard 209A; that is less than 100 particles of sizegreater than 0.5 microns per cubic foot and none of 5 microns and acheck of the apparatus showed that air delivered to the clean area didnot contain more than particles of a size greater than 0.5 microns percubic foot.

STARTING MATERIAL The material used in this work was polyacrylonitrilewhich included 6% by weight of methyl acrylate as comonomer and had anumber average molecular weight of 52500.

The spinning solution was prepared by dissolving 14 weight of thepolyacrylonitrile/methyl acrylate copolymer in a 50 weight aqueoussodium thiocyanate solvent at a temperature of 9095C. The viscous copolymer solution was stirred for about an hour and while still hot waspassed through a 1.5 micron filter.

The solution was then de-aerated by warming to a temperature of about60C, and centrifuging in 3 inch tubes, in an 8 inch diameter centrifugeat 4000 evolutions per minute. The coagulant bath contents, a 10% byweight aqueous sodium thiocyanate, solution, and the distilled waterused in the wash baths and in the steam generators were filtered through0.25 micron filters using a peristaltic pump to provide the drivingforce.

SPINNING The spinning solution, obtained as described above, at roomtemperature was spun through a five hole spinneret having 75 micronholes in a 10% by weight aqueous sodium thiocyanate coagulation bath atat extru sion rate of 0.30 metres/minute and the speed at the firstroller was 0.60 meter/minute. The temperature of the first wash bath was50C, the steam stretch ratio was 14 and the final wash bath temperaturewas 30C.

CONVERSION TO CARBON FIBRES 5 The polyacrylonitrile precursor fibre wassecured to the collecting frame so that it could not shrink duringoxidation and was oxidised at a temperature of 220C for 8 hours in aglass vessel in the clean zone. Oxygen was passed into the oxidation rigthrough a 0.05 micron 0 filter. 1::

Further processing to carbon fibre was carried out stepwise, firstcarbonisation at 1000C in a nitrogen atmosphere while the fibres werestill on the fused silica collecting frame then further heat treatmentto 1400C 15 in a vacuum furnace or to '2500C in an argon atmosphere in acarbon tube furnace.

The oxidised fibre while still on the fused silica collection frame wasplaced in a fused silica tube with a tightly fitting cap while in theclean zone to prevent contamination. The tube was then transferred to afurnace and the oxidised fibre heated to 1000C for a period of /2 hourto carbonise the fibres. During the heating a stream of filterednitrogen was passed over the fibres.

At this stage the sample was split in two and each sample placed in aclose fitting carbon tube. One sample was heat treated at 1400C in avacuum furnace for /2 hour and the other at 2500C in a carbon tubefurnace for /2 hour in a steam filtered argon.

The carbonising and heat treatment furnaces were not in the clean zonebut all transfers were carried out in the clean zone to prevent, or atleast minimise, surface contamination during transfer. The streams ofnitrogen and argon used were filtered by passing them through 0.05micron filter Control samples, using the same filtered spinning solutionwere spun on similar apparatus to the same parameters, but not in theclean zone and were then carbonised and further heat treated in exactlythe same way as described above.

TESTING OF CARBON FIBRES The properties of the carbon fibres produced,measured as the average of 20 determinations on 5 cm gauge lengths ineach case, are given in Table 1 below and illustrated in FIG. 1.

TABLE 1 Fibre Properties Clean zone Control treatments fibres fibresDiameter, microns 15.9 15.0 Polyacrylonitrile Elongation, 1O 1 l fibresYoungs modulus, psi 1.45 X 10 1.88 X l0 as Tensile strength, psi 79.7 X10" 92.2 X 10" spun Coefficient of variation of strengths, 8 15Diameter, microns 8.2 7.5 carbonized Youngs modulus psi 26.9 X 10" 24.9X 10 in nitrogen Tensile strength, psi 318 X 10 282 X 10" V2 hour atCoefficient of variation 1000C of strengths, 15 32 Heat treatmentDiameter microns 7.8 7.0 1400C Youngs modulus, psi 31.0 X 10' 29.0 X 10in vacuum Tensile strength, psi 349 X 10'' X 10" V2 hour at Coefficientof variation 1400C of strengths, 7r 14 33 Heat Treatment Diametermicrons 7.5 6.3 in argon Youngs modulus, psi 55.1 X l0 53.1 X 10 V2 hourat Tensile strength, psi 400 X l0" 245 X 10" 2500C Cocfficicnt ofvariation 27 33 of strengths, 71

In the accompanying FIG. 1 the line 1 represents carbon fibres producedby the process of the present invention whereas line 2 represents carbonfibres produced by an identical process except that the fibres, althoughnot deliberately contaminated, where not spun in clean conditions. Thedotted lines represent the 95% confidence limits of the quoted results.Line 2 clearly shows a maximum in ultimate tensile strength, which isnormally found whereas line 1 shows that the carbon fibre produced inaccordance with the present invention has an ultimate tensile strengthwhich increases as the final heat treatment temperature increases.

In interpreting these results and in particular in comparing them withprior art results it should be noted that the careful exclusion ofcontamination the form of particles from the spinning solution andliquids used therein and from the oxygen permeation, carbonisation andfurther heat treatment processes applied to the control fibres in theseexperiments has not been general practice in the prior art. It shouldalso be noted that there is evidence for the existance of a scale effectby which improved absolute results are obtained by increasing thequantities of fibre treated. For example, the fibres for the presentexperiments were spun from a spinneret having 5 holes and 0.4g weretreated and the control fibres showed a tensile strength maximum in theregion of I000C. Commercial fibre tows have in general many morefilaments. For example carbon fibres from 10,000 filament Courtelle showa tensile strength maximum in the region of I500C. However the trends oftensile strength are not affected by scale only the absolute values.

A series of tests using gauge lengths of 2.5 cm and 1.0 cm were carriedout on the fibres produced by the pro cess of the present invention andon the control fibres and the results are given in Table 2 below andplotted as a graph in accompanying FIG. 2.

TABLE 2 invention to show any gauge length effect.

It will of course be realised that although the present invention isspecifically described in this specification in terms of discrete steps,the invention, the subject of the present application, is readilyadapted to processes for the continuous production of carbon fibres.

What we claim is:

1. A process for the preparation of polyacrylonitrile precursor fibresand their subsequent conversion to carbon fibres which includes thesteps of spinning the polyacrylonitrile precursor fibres from solution,heating the precursor fibres at a temperature in the range 200300C in anoxidising atmosphere for a time sufficient to permit complete permeationof oxygen while the natural shrinkage of the fibre is at leastrestrained, followed by carbonisation and further heat treatment attemperatures of up to 3000C which includes the improvement wherein thespinning process and oxygen permeation process are carried out underconditions in which particles, and in the case of the spinning solution,gas bubbles, are excluded from the liquids employed in the spinningprocess and from the gases in which such process and the oxygenpermeation process take place, whereby carbon fibres are produced havingan ultimate tensile strength which increases over the whole range offinal heat treatment temperatures.

2. A process as claimed in claim 1 in which the particles are excludedby filtering the said liquids and gases through filters capable ofremoving any particles having a size greater than 3 microns.

3. A process as claimed in claim 2 wherein the filters are capable ofremoving any particles having a size greater than 1.5 microns.

4. A process as claimed in claim 3 in which the said gases are filteredthrough filters capable of removing any particles having a size greaterthan 0.05 microns.

5. A process as claimed in claim 4 and in which the Clean zone fibresControl fibres Gauge Diameter Strength Coefficient Diameter StrengthCoefficient length psi of microns psi of cm X 10" variation X It)variation L0 7.9 397 I3 6.3 3l8 25 2.5 7.5 400 18 6.3 259 32 5.0 7.5 39927 6.3 245 33 In accompanying FIG. 2 line 1 represents the resultsobtained from carbon fibres obtained by the process of the presentinvention, line 2 represents the control fibres and line 3 representsresults obtained from carbon fibres obtained from a proprietaryprecursor.

These last results are taken from a paper, The effect of gauge length ofthe tensile strength of carbon fibres by R Moreton appearing in FibreScience Technology, I, 4, 273(1969).

It is believed that breakages in tensile test samples are caused byrandom faults in the fibres. The theory hasit that the longer the gaugelength the greater the liklihood of including a fault and therefore thelower the average tensile strength. This is borne out by lines 2 and 3in FIG. 2 but line 1 may be interpreted as an indication that grossfaults of the type causing failure in the control fibres do not occursufficiently frequently in carbon fibres produced by the process of thepresent carbon fibres shown no significant gauge length effect whensubjected to tensile strength testing at lengths of 5 cm or less.

6. A process as claimed in claim 5 in which the polyacrylonitrilecontains less than 10% by weight of other monomers.

7. A process as claimed in claim 1 in which the carbonisation andfurther heat treatment are also carried out under conditions in whichparticles are excluded.

8. A process as claimed in claim 7 in which the particles are excludedby filtering the said liquids and gases through filters capable ofremoving any particles having a size greater than 3 microns.

9. A process as claimed in claim 8 wherein the filters are capable ofremoving any particles having a size greater than 1.5 microns.

10. A process as claimed in claim 9 in which the said gases are filteredthrough filters capable of removing cm or less.

sting at lenths of 12. A process as claimed in claim 11 in which thepolyacrylonitrile contains less than 10% by weight of other monomers.

UNITED STATES PATENT OFFICE Q TEFICATE OF QORRECTEN PATENT NO. 1 3,904,716

DATED September 9 1975 INVENTOMS) 1 Roger Moreton and William Watt it iscertified that error appears in the above-identified patent and thatsaid Letters Patent Q are hereby corrected as shown below:

Column 1, Line 54 between "also" and "out read carried..

0 Column 6, Line 50 read "shown" as show--.

Etigncd and fizalcd this v t twenty-third Of March 1976 Q [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Aflesfing ff (ummr'ssr'rmer uflalenls andTrademarks

1. A PROCESS FOR THE PREPARATION OF POLYACRYLONITRILE PRECURSOR FIBRESAND THEIR SUBSEQUENT CONVERSION TO CARBON FIBRES WHICH INCLUDES THESTEPS OF SPINNING THE POLYACRYLONTRILE PRECURSOR FIBRES FROM SOLUTION,HEATING THE PRECURSOR FIBERS AT A TEMPERATURE IN THE RANGE 200#-300#C INAN OXIDISING ATMOSPHERE FOR A TIME SUFFICIENT TO PERMIT COMPLETEPERMEATION OF OXYGEN WHILE THE NATURAL SHRINKAGE OF THE FIBRE IS ATLEAST RESTRAINED, FOLLOWED BY CARBONISATION AND FURTHER HEAT TREATMENTAT TEMPERATURES OF UP TO 3000#C WHICH INCLUDES THE IMPROVEMENT WHEREINTHE SPINNING PROCESS AND OXYGEN PERMEATION PROCESS ARE CARRIED OUT UNDERCONDITIONS IN WHICH PARTICLES, AND IN THE CASE OF THE SPINNING SOLUTION,GAS BUBBLES, ARE EXCLUDED FROM THE LIQUIDS EMPLOYED IN THE SPINNINGPROCESS AND FROM THE GASES IN WHICH SUCH PROCESS AND THE OXYGENPERMEATION PROCESS TAKE PLACE, WHEREBY CARBON FIBRES ARE PRODUCED HAVINGAN ULTIMATE TENSILE STRENGTH WHICH INCREASES OVER THE WHOLE RANGE OFFINAL HEAT TREATMENT TEMPERATURES.
 2. A process as claimed in claim 1 inwhich the particles are excluded by filtering the said liquids and gasesthrough filters capable of removing any particles having a size greaterthan 3 microns.
 3. A process as claimed in claim 2 wherein the filtersare capable of removing any particles having a size greater than 1.5microns.
 4. A process as claimed in claim 3 in which the said gases arefiltered through filters capable of removing any particles having a sizegreater than 0.05 microns.
 5. A process as claimed in claim 4 and inwhich the carbon fibres shown no significant gauge length effect whensubjected to tensile strength testing at lengths of 5 cm or less.
 6. Aprocess as claimed in claim 5 in which the polyacrylonitrile containsless than 10% by weight of other monomers.
 7. A process as claimed inclaim 1 in which the carbonisation and further heat treatment are alsocarried out under conditions in which particles are excluded.
 8. Aprocess as claimed in claim 7 in which the particles are exCluded byfiltering the said liquids and gases through filters capable of removingany particles having a size greater than 3 microns.
 9. A process asclaimed in claim 8 wherein the filters are capable of removing anyparticles having a size greater than 1.5 microns.
 10. A process asclaimed in claim 9 in which the said gases are filtered through filterscapable of removing any particles having a size greater than 0.05microns.
 11. A process as claimed in claim 10 and in which the carbonfibres show no significant gauge length effect when subjected to tensilestrength testing at lenths of 5 cm or less.
 12. A process as claimed inclaim 11 in which the polyacrylonitrile contains less than 10% by weightof other monomers.